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Probing Question: Could Mars ever be made habitable for human beings?

David Pacchioli

October 10, 2005

Probing Question: Could Mars ever be made habitable for human beings?

Terraforming Mars—making the Red Planet Earth-like, so that humans could live there—is an idea that has been around since at least the early 1970s, when Carl Sagan and others first raised it seriously. In the 1990s, science-fiction writer Kim Stanley Robinson based his 1,600-page Mars trilogy (Red Mars, Green Mars, Blue Mars) on the concept.

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Not much atmosphere: Could the Red Planet ever warm up to humans?

In addition to being a good story, Robinson's epic is well-grounded in the current science, says Darren Williams, associate professor of physics and astronomy at Penn State Erie. "There's actually been a good bit of research done into this question," Williams says.

Some of that work is by Williams' former adviser at Penn State, distinguished professor of geosciences James Kasting. In a 1991 paper published in the journal Nature, titled "Making Mars Habitable," Kasting and colleagues Christopher McKay and Owen Toon outlined the necessary steps.

First, the planet's surface temperature would have to be boosted from its current frigid -76 degrees F. No need to go as high as Earth's average of 60 degrees F, Williams says, "but you have to make it warm enough to support the presence of liquid water." At the same time, you'd need to thicken the wispy Martian atmosphere enough to hold that heat in and prevent the water from instantly evaporating. Also, you'd need an equivalent of Earth's ozone layer to shield any surface life against the lethal effects of the Sun's ultraviolet rays.

The only encouragement in this daunting "to do" list is that the tasks are interconnected. Start on one and you start them all. And, the thinking goes, it might take only a relatively small stimulus to throw a much larger process into motion.

As Williams explains, "There could be lots of carbon dioxide present on Mars, both frozen in the planet's polar ice caps and tied up in the top layer of surface rock, the regolith." If the planet's surface could be warmed just enough to begin melting that ice, that carbon dioxide, released into the atmosphere, would begin to act as a greenhouse gas. Its presence would raise the atmospheric pressure and hold in enough heat to cause sub-surface melting—releasing more carbon dioxide and setting up a positive feedback loop that would continue to play until the system eventually stabilized.

One way to trigger that initial warming, futurists have suggested, would be to deploy giant mirrors in space, redirecting sunlight toward the Martian poles. Another would be to sprinkle the ice caps with a dark-colored dust, increasing their heat-absorption. (Kind of like throwing fireplace ashes on a winter sidewalk.) A third way would be to inject chlorofluorocarbons—potent greenhouse gases—into the Martian atmosphere. Continually replenished in sufficient amount, the thinking goes, these CFCs could provide a blanket of insulation while also acting as a shield against incoming ultraviolet rays.

Kasting himself throws doubt on this scenario. "CFCs break down pretty rapidly," he notes. "On Earth, their lifetime is about 100 years, but in an atmosphere without oxygen and ozone, it would be much shorter. And they don't make a good shield, because the UV light is continually breaking them down." More basically, he adds, "I don't think there's enough carbon dioxide on Mars to effect much warming. We may be missing something, but if it's there we haven't found it."

Even if the "runaway greenhouse" theory could be realized, he says, it would take over 100 years to render Mars a viable habitat—and then only for plants. Making it fit for humans would be a far more complicated project.

Plants can get by on very little atmospheric oxygen, Kasting explains—probably not a whole lot more than Mars's current 0.15 percent—while humans need close to 20 percent. To make up that difference you'd have to rely on large-scale photosynthesis. And in order to produce net oxygen from all those plants, you'd need a way to absorb some of the carbon released when they die and decay. "On Earth," notes Kasting, "that crucial fraction of carbon gets buried in the oceans."

You'd also need to bring in a "background gas" like nitrogen, which makes up 78 percent of our air. The best hope for doing that, Kasting says, would be to engineer a collision between Mars and a large, nitrogen-rich comet—something like the impacts that are supposed to have created Earth's atmosphere. "But that would require a technology that may be a thousand years down the road."

"I think it would be almost impossible to make Mars habitable for humans," Kasting offers at last. "It would be easier to do for plants, but I don't know why we'd want to."

"I think we will have humans there some day," he adds. "But they'll be in domed cities, like in the movie Total Recall. They'll be scientific outposts."

"To me, the most interesting thing about Mars would be to go there, not to disrupt it, but to go there and look for life. There's all sorts of interesting science we could do."

James F. Kasting, Ph.D., distinguished professor of geosciences in the College of Earth and Mineral Sciences, can be reached at jfk4@psu.edu. Darren M. Williams, Ph.D., associate professor of physics and astronomy at Penn State Erie, can be reached at dmw145@psu.edu.